Members of the genus chlamydia are obligately intracellular parasitic bacteria which cause various diseases of animals and man. The purpose of this proposal is to investigate the mechanisms by which chlamydiae attach to and gain entry into host cells. Studies will include investigation of the surface charge on C. trachomatis by cell cytophroesis and isoelectric focussing as well as the relative hydrophobicity of the surface of C. trachomatis by hydrophobic interaction chromatography. Various parameters of attachment of C. trachomatis to unfixed and formaldehyde fixed L-cells will be studied, including the effect of cations, non-ionic detergents, and various reagents which alter the surface properties of both the host and the parasite. L-cell receptors for C. psittaci will be isolated by radio-labeling the surface of L-cells, isolating their membranes, solubilizing membrane proteins with non-ionic detergent, mixing host membrane proteins with C. psittaci, and co-purifying host material with C. psittaci. C. psittaci receptors for L-cells will be isolated by labeling the surface of C. psittaci, isolating outer membranes and solubilizing outer membrane proteins with SDS, mixing this material with formaldehyde-fixed L-cells and characterizing the parasite material which adheres to host cells.